The objectives of the proposed research are to determine the physiological mechanisms which regulate creatine biosynthesis during development of the mammalian fetus. In order to accomplish these objectives we intend to identify the factors which promote the development of the enzyme which catalyzes the synthesis of creatine, namely, S-adenosylmethionine:guanidinoacetate N-methyltransferase (GA-methyltrasferase) in the fetal liver of the rat and guinea pig. Appropriate hormones such as thyroxine, glucagon, hydrocortisone and insulin as well as creatine precursors will be administered to intact fetuses in utero and after 12-24 hours the GA-methyltransferase activity of fetal liver will be assayed in order to determine if the hormonal stimulus can evoke the premature formation or accelerate the accumulation of the enzyme. Metabolic inhibitors (puromycin and actinomycin D) will be administered to the fetuses in utero along with appropriate hormones (and other effectors) to determine if observed changes in fetal liver GA-methyltransferase activities are dependent on protein and/or RNA synthesis. An immunochemical assay for GA-methyltransferase will be used to determine if a net increase in protein synthesis can account for changes observed in fetal liver GA-methyltransferase activities after hormonal treatment. Organ cultures of fetal liver explants will be assayed for GA-methyltransferase activity after incubation with/and without hormones and metabolic inhibitors to determine the mechanism of action of the various effectors at the cellular and molecular level.